Thu, December 26, 2019 Turnagain Pass Avalanche Forecast

Thu, December 26th, 2019

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

2 - Moderate

0 - No Rating

1 - Low

2 - Moderate

3 - Considerable

4 - High

5 - Extreme


Travel Advice	Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.	Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.	Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential.	Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.	Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches	Natural and human-triggered avalanches unlikely.	Natural avalanches unlikely; human-triggered avalanches possible.	Natural avalanches possible; human-triggered avalanches likely.	Natural avalanches likely; human-triggered avalanches very likely.	Natural and human-triggered avalanches certain.
Avalanche Size and Distribution	Small avalanches in isolated areas or extreme terrain.	Small avalanches in specific areas; or large avalanches in isolated areas.	Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.	Large avalanches in many areas; or very large avalanches in specific areas.	Very large avalanches in many areas.

Avalanche Problem 1

Wind Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Likelihood of Avalanches

Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches

Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word	Size (D scale)	Simple Descriptor
Small	1	Unlikely to bury a person
Large	2	Can bury a person
Very Large	3	Can destroy a house
Historic	4 & 5	Can destroy part or all of a village

More info at Avalanche.org

The region has picked up 6″ to a foot of new snow in the last 24 hrs. The winds were easterly and sustained 10-20 mph with gusts into the 40s. Today a few more inches of snow are forecast to fall. The winds have shifted to the west/northwest and look to be strong enough to transport snow until this evening. Expect wind slabs on steep leeward slopes and in gully features exposed to the wind to be easily triggered by the weight of a person. Because of the continued active loading there is also a chance of natural avalanches today. Be mindful of the terrain above you. The new snow covered a layer of surface hoar and there is another buried layer a few inches deeper. These both have the potential to be reactive especially as the slab above becomes more cohesive with wind effect. Choose terrain carefully and look for signs of instability. What happens when you are breaking trail or if you step off the skin track? Cracking? Whumpfing?

Loose Snow Avalanches and Storm Slabs: On protected steep slopes watch for sluffing and/or small storm slabs. Does the snow seem light and fluffy and move with you or does the recent snow seem slightly stiffer than the snow below and start cracking? How well is the new snow bonding to the snow below? Hand pits and riding very small test slopes are a good way to look for storm slabs.

Cornices: Active wind transport will also make cornices larger, increase the chance of natural cornice falls and the potential to trigger them if you get too close. The shifting wind direction may also make new cornices in the opposite direction and make ridgeline travel more complex. Pay attention to loading patterns.

Snow piling up at road level on Turnagain Pass.

Buried Solstice surface hoar in a pit at 2100′ on Seattle Ridge, 12.24.19. Photo: Trip Kinney.

Obscured skies and gusty winds on Tincan yesterday, 12.25.19. Photo: Kakiko Ramos-Leon

Avalanche Problem 2

Glide Avalanches
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Glide Avalanches

Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

Likelihood of Avalanches

Size of Avalanches

Signal Word	Size (D scale)	Simple Descriptor
Small	1	Unlikely to bury a person
Large	2	Can bury a person
Very Large	3	Can destroy a house
Historic	4 & 5	Can destroy part or all of a village

More info at Avalanche.org

As glide cracks fill in with new snow and wind-loading they may easily be forgotten. It will be important to look for them in the terrain as you move through. They may open and become more obvious again as skies clear and temperatures cool overnight. Remember glide avalanches are very unpredictable and that it’s not a good idea to linger under glide cracks.

Additional Concern

Persistent Slabs

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

More info at Avalanche.org

In areas where the snow pack is shallower like Summit Lake, the south end of Turnagain and in the Crow Pass terrain north of Girdwood we have been tracking a weak layer of snow sitting near the base of the snowpack in the Alpine. There has been a lingering concern a person could trigger a large avalanche if they hit the wrong thin spot. Overall this layer appears to be dormant. However, with the active weather forecast into next week we will be monitoring potential reactivity with further loading.

Weather

Thu, December 26th, 2019

Yesterday: Skies were cloudy and snow fell throughout the day and into the evening. Winds were easterly 10-20 mph gusting into the 40s. Temperatures were in the teens to mid 20°Fs. Overnight winds shifted to the west and light snow continued.

Today: Cloudy skies and snow showers in the morning. Another 1-3″ of snow is forecast. Temperatures will be in the teens to mid 20°Fs today. As skies begin to clear in the late afternoon temperatures will start to drop into the low teens and then down to the single digits overnight. Winds will be westerly 10-20 mph with gusts into the 40s. These should ease off early tomorrow morning.

Tomorrow: Mostly sunny skies in the morning with clouds increasing in the afternoon. Temperatures in the teens. Winds will be light and westerly. Snow starts again overnight into Saturday as the next storm moves into the area. The area looks to be in an active weather pattern into the New Year.

PRECIPITATION 24-hour data (6am – 6am)

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	24	8	0.6	37
Summit Lake (1400′)	22	1	0.1	11
Alyeska Mid (1700′)	24	6	0.49	27

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	16	NE	13	49
Seattle Ridge (2400′)	NA*	NA*	NA*	NA*

*Rime has covered the wind sensor on Seattle Ridge.

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
10/27/24	Turnagain	Observation: Tincan	Michael Kerst
10/21/24	Turnagain	Observation: Turnagain Pass Road Observation	Trevor Clayton
10/19/24	Turnagain	Observation: Tincan – Below Todds Run	Andy Moderow
10/18/24	Turnagain	Observation: Taylor Pass	Eli Neuffer
10/15/24	Turnagain	Observation: Tincan Common	John Sykes Forecaster
10/14/24	Turnagain	Avalanche: Tincan	CNFAC Staff
05/13/24	Turnagain	Observation: Eddie’s, Sunburst, Seattle, Cornbiscuit, Pete’s South	H Thamm
05/13/24	Turnagain	Observation: Turnagain Pass non-motorized side	Amy Holman
05/12/24	Turnagain	Observation: Warm up Bowl	Tony Naciuk
05/07/24	Turnagain	Observation: Turnagain Pass Wet Slabs	A S

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